The invention relates to inertial filters to be disposed in an air flow path that are configured to classify submicron or smaller particles contained in a stream of air flowing in the air flow path, particularly nanoparticles having particle sizes equal to or smaller than 100 nm, by utilizing the effect of inertial impaction.
A conventional inertial filter 100 is described below referring to FIG. 11. The conventional inertial filter 100 is disposed in an air flow path to classify particles. The conventional filter 100 has a cylindrical filter body 101, and the filter body 101 has a through cavity 102 having a circular shape in cross section and penetrating through from the upstream side to the downstream side of an airstream passage. The through cavity 102 includes a diametrically-reduced through cavity 102a and a diametrically-constant through cavity 102b. The diametrically-reduced through cavity 102a is provided on the upstream side of the airstream passage and has an inner diameter progressively smaller. The diametrically-constant through cavity 102b is continuous to the diametrically-reduced through cavity 102a on the downstream side of the airstream passage and has an inner diameter dimensionally fixed. The diametrically-constant through cavity 102b is filled with a metal fiber 103 which is an example of incompressible fibers. The metal fiber 103 is secured by a mechanism not illustrated in the drawing so that the metal fiber 103 does not fall off from the diametrically-constant through cavity 102b and drop downward in an airstream passing direction.
In the inertial filter 100, its internal pressure is lowered to or under an external pressure by a suctioning force of a pump not illustrated in the drawing, so that a stream of air is generated in the through cavity 102 by a pressure difference between the internal and external pressures in a direction from an arrow A toward an arrow B illustrated in the drawing to allow for classification of particles. The pressure difference is generated by lowering the internal pressure from the external pressure. The stream of air increases its flow velocity in the diametrically-reduced through cavity 102a, and the flow velocity becomes constant once the stream of air enters the diametrically-constant through cavity 102b. Then, fine particles contained in the stream of air is collided with and captured (collected) by the metal fiber 103 in the diametrically-constant through cavity 102b. 
Background prior art includes JP Patent Application Publication No. 2008-70222.